Conf.py 23.08 update

[jarmak-nv]

This PR does two things to conf.py

1. Updates the Docker pull commands to match the RAPIDS 23.08 updates
2. Updates the script so we only need to update the versions in 1 place to reduce human error possibility

It also updates the versions for 23.08 since we'll need to do that prior to release anyway.

[jacobtomlinson]

Thanks @jarmak-nv. I pushed an extra commit to restore the -nightly suffix to rapids_version. It looks like we only use that variable in coiled.md right now and it makes more sense to name the environment XX.XX-nightly so that it is clear when we switch to a stable one.